Wind turbine generators are regarded as environmentally friendly and relatively inexpensive alternative sources of energy that utilize wind energy to produce electrical power. A wind turbine generator generally includes a wind rotor having turbine blades that transform wind energy into rotational motion of a drive shaft, which in turn is utilized to drive a rotor of an electrical generator to produce electrical power. Modern wind power generation systems typically take the form of a wind-farm having multiple such wind turbine generators that are operable to supply power to a transmission system providing power to a utility system.
These wind turbine generators and wind-farms are typically designed to deliver power to the utility system with the power being independent of system frequency. Some wind turbine generators have a variable frequency operation and require a variable frequency power electronic inverter to interface the wind turbine generator output with the utility grid. In one common approach, the wind turbine generator output is directly fed to a power electronic converter, where the turbine frequency is rectified and inverted into a fixed frequency as needed by the utility system. An alternative approach uses a doubly fed asynchronous generator (DFAG) with a variable frequency power electronic inverter exciting the DFAG rotor and stator windings being coupled directly to the utility system.
In traditional power systems, the frequency of the synchronous generators of the power system match the utility system, and the dynamic response of the frequency of the utility system is dependent upon the inertia of the synchronous generators and loads. Synchronous generators used in a traditional power system are able to contribute in frequency and voltage control of the power system during transient conditions, that is, sudden failure of generation, line fault or connection of a large load. During such transient conditions, the system frequency starts to change at a rate mainly determined by the total angular momentum of the system, which is a sum of the angular moment of all the generators and rotating loads connected to the power system. In such transient conditions, the synchronous generators may also provide additional control services that modulate active power to stabilize the power system and restore frequency to its nominal value.
Wind turbines, when used for generating power in a power system, however, do not generally contribute to the frequency stabilization of the utility system. As more power generated by wind turbines is interfaced through the utility system, it would be desirable for wind turbines to also contribute to the voltage and frequency control of the power system in transient conditions in order to stabilize the power system.
Therefore there is a growing need to overcome the above mentioned limitations for wind turbine systems and to provide control techniques so that the wind turbines can participate more intelligently in frequency regulation and power-swing stabilization for the utility system.